GCG and metabolic syndrome: The causes of metabolic syndrome early improvement are complex, being probably connected with anatomical changes, such as duodenal exclusion and overflow of nutrients to the distal small bowel, and with the subsequent variations of the entero-insular axis mediated by gastrointestinal incretins (i.e., glucagon-like peptide 1 (GLP-1), and gastrointestinal insulinotropic polypeptide (GIP)) [130,131,132].